Archives of
Arch. Otorhinolaryngol. 222, 205-209 (1979)
Oto-Rhino-Laryngology 9 Springer-Verlag1979
The Origin of the Waveform of Cochlear Whole Action Potential H. J. J. Boelen Joris v. d. Haagenlaan 20, Arnhem, The Netherlands Summary. An explanation is given why the recorded action potentials of the cochlea manifest themselves in the complex N1N2-form. The separate AP of the nerve fibers are not diphasic but triphasic in form. With algebraic addition of the, in different phase, AP present in the auditory nerve the N1N 2 complex originates. This is illustrated by the observed form changes in the recorded signals during the cutting experiments of the auditory nerve. Key words: N1N 2 complex of the cochlea
Although action potentials in the cochlear nerve were shown by Buytendijk as early as 1911, there is still no communis opinio as to why the summated AP manifests itself in the characteristic N1N:form. Katsuki and Davis (1954) explained N 2 by the plural response of the nerve: the response with more than one spike of the separate nerve fibers of the cochlear nerve to a presented click. Sorensen (1959) thought that N 2 was mainly formed by the cochlear nucleus in the pons. Rosenblith and Rosenzweig (1951) found a discontinuity in the increase of the amplitude of N 1, recorded from the total cochlear nerve as a function of the rise intensity of the presented signal. They developed the second population theory. Spoendlin (1971) made plausible a different activity of the inner and outer hair cells of the organ of Corti on the basis of his electron microscopic investigations. According to him, the outer hair cells and the afferent nerve fibers connected form part of a very sensitive system, in which summation of pulses of about 10 outer hair cells may occur in one afferent neuron. Sachs and Kiang (1968), however, demonstrated that all afferent nerve fibers of the cochlear nerve recorded by them reacted to stimuli being close to the auditory threshold as to their level of intensity. Teas et al. (1962) and Legouix et al. (1978) presented a model of the AP to explain how diphasic neural elements of the population of the fibers could combine
0302-9530/79/0222/0205/$ 01.00
206
H.J.J. Boelen
in amplitude and phase as a travelling wave excites the elements successively to produce the classical N 1 and N 2 deflections. Boelen (1976), however, demonstrated that the AP in the separate nerve fibers of the cochlear nerve are not diphasic but triphasic. In measurements at the auditory nerve with guinea pigs there is still found a positive peak Pl before the first negative deflection N 1. This P1 is the first phase of the AP "the approaching wave" and it is formed by the positive mutation of polarity in the nerve as an introduction of the negatively directed membrane shifts. Finally, as third phase, the positive recovery phase of the nerve fibers follows. The recorded AP are formed in the well myelinated part of the cochlear nerve. The myelin sheaths of the afferent neurons connected to the inner hair cells already start at the habenula perforata, before the bipolar ganglion cells. The longer the distance covered by the approaching wave in these myelin sheaths up to the recording electrode, the larger the recorded Pj becomes. The P~ is to be recognized clearly in the signals recorded directly from the auditory nerve. A critical examination of the signals recorded from the cochlea shows the PI occasionally, the amplitude of the Pa being much smaller though. At the cochlea recording, the recording electrode is directly situated at the spot of origin of the AP's arising in the underlying turn. Only AP's stimulated by hair cells of cochlea turns situated higher, cover a small distance up to the recording electrode and are able to show some of their approach in the form of a P1. The basilar membrane with human beings is twice as long as it is with guinea pigs, though the part of the auditory nerve situated intracochlear being longer, too. On examination of electrocochleograms recorded with human beings, the P1 "the positive approaching wave" must be taken into account (Boelen, 1979). In order to explain the N1N z complex stimulated by a presented click and recorded from the cochlea, we will have to make use of the following four electrophysiological phenomena: i. the summation of the spikes present in the cochlear nerve; 2. the triphasic phenomenon of the separate spikes with its two positive peaks and one negative deflection; 3. the difference in phase of the spikes present, caused by the travel time of the pulse wave along the basilar membrane; 4. as to the P~, the distance already covered by the several AP in the myelinated nerve fibers up to the recording electrode. The AP recorded in a node of Ranvier of a separate nerve fiber have a size of at least 75 inV. Recordings of the whole cochlear nerve show 0.6 mV after 1000 x preamplification. The algebraic addition of the various positive and negative phases of the AP makes the recorded electric phenomenon more complex but much smaller at the same time. An experiment, in which the auditory nerve of a guinea pig is cut, will demonstrate that the summation of spikes define the form of the N~N 2 complex.
Experiment In an experiment,elaboratelydescr/bedin the afore mentionedpublications,there is a recordingwith guineapigs with two recordingelectrodessimultaneoustyfrom the cochlea and the cochlearnerve. The
The Origin of the Waveform of Cochlear Whole Action Potential
207
cochlea electrode was placed via the opened bulla on the wall of the scala tympani of the second turn. Via a bore hole at the base of the cochlea a tunnel is drilled to the meatus acusticus internus. A needle electrode is placed via this tunnel in the auditory nerve with the aid of a microelectrode holder. Several times this needle electrode is removed and with a fine knife the cochlear nerve can be cut in stages until all nerve fibers have been cut and there can be recorded from the periphere stump only. The earth
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.
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Fig. 1. Signals recorded from the cochlea and the cochlear nerve during and after the cutting of this nerve. In the figure the signals of the cochlea are represented on the left side, on the right side the accessory signals of the periphere part of the cochlear nerve. Time basis 1 ms, the amplification and the intensity of the presented click are mentioned in the relating signal. Picture 1 taken before the cutting, picture 2 and 3 during the cutting, picture 4 and 5 after eompiete cutting of the nerve, picture 6 indicates the moment before the presented click
208
H.J.J. Boelen
electrode was placed in the neck muscles. During the recordings the experimental animal is presented a click via a condensator telephone every second. The preampliflcation is 1000• In order to get a better survey of the recordings, the recordings of the cochlea and the cochlear nerve, taken simultaneously, are not projected underneath each other but next to each other. Out of the series of signals of a cutexperiment only five pairs of recordings have been pictured (Fig. 1). Viewing the changes in the recorded signals from the cochlea and the cochlear nerve during the cutting of the cochlear nerve we observe: - In the cochlea recording the N: is gradually being taken into the N 1, the basis of the NIN2 complex getting broader. - In the cochlear nerve positive peaks develop, finally the total recorded signal becomes positive. - After complete cutting of the nerve the N1 of the cochlea-recording is strongly enlarged. - The period of time between the beginning of the stimulus and the maximal deflection of the N1 has increased with 0.25 ms.
Discussion After the cutting of the cochlear nerve the maximum amplitude of the N~ in the signal recorded from the cochlea is reached 0.25 ms later than before the cutting. At the same time this amplitude has become 50 times as large at, e.g., a click of 60 dB. The period of time, represented in the basis of the recorded signal, is lengthened as well. These phenomena are to be explained by a strong decrease of the influence of the positive recovery phases of the spikes on the total signal. A n A P is an all or nothing phenomenon. The negative membrane shifts are maximal. After the cutting, however, the recovery phases will pass less quickly in the heavily damaged auditory nerve and thus be directed less strongly positive. The separate nerve fibers are still able to react. The deflection o f the N x, however, formed by the negatively directed membrane shifts originating at the beginning of a click and increasing in number in conformity with the travel time of the pulse wave along the basilar membrane, is later and less strong levelled by the very weakened positive recovery phases of the AP. The N 1 now can manifest itself longer and much more extensively. In the algebraic addition the in amplitude reduced positive recovery phases count less. The N 2 - partially formed by the plural response of the nerve fibers - has been shifted by the extensive N~ and taken into it.
Conclusion With the above mentioned explanation an answer can be given to the origin and the form of the N~N 2 complex "recorded from the cochlea".
Firstly: the
latency period up to the beginning of the N 1 is formed by the period o f time the sound wave needs to reach the hair cells on the beginning of the basilar membrane to be stimulated and by the chemical synaptic transmission between hair cells and afferent neurons.
Secondly: the
two deflections N 1 and N 2 form part o f one big deflection originating by the negative membrane shifts in the afferent neurons of the cochlear nerve in the cochlea. This big deflection will reach its maximum after over 0.25 ms after the
The Origin of the Waveform of Cochlear Whole Action Potential
209
normally found N1 in order to disappear in about 2.5 ms, according to the intensity of the presented click. The soundwave in the scalae needs over 3 ms to reach the hair ceils in the last turn. 1 ms after the beginning of N~ a small increase of negative membrane shifts can be expected, because of the second response of the first neurons after their refractory periods.
Thirdly: because of the contrary influence of the P1 "the positive approaching wave", in the algebraic addition by the recording electrode on the N1, the amplitude of the N~ is represented smaller. The effect of the P~ on the N~ is to be expected only in the second part of the descending line of the N~, when the lower frequency sectors of the basilar membrane are stimulated. N o t until then the action potentials have covered a small distance to the recording electrode and P1 can develop. In a recording of the auditory nerve the P1 will be clearly visible. Fourthly: by
adding the influence of the recovery phases of the directly generated A P to the approaching wave of the action potentials of the lower frequency areas a peak - P2 - originates in the big deflection of the negative membrane shifts between the N 1 and N 2. Owing to this the N 1 and N 2 of the cochlea recording get their characteristic form.
References Boelen, H. J. J.: Her ontstaan van de aktiepotentialen N~ en N 2 in het gehoororgaan, met een verklaring voor her vinden van een P~ in her electrocochleogram. Thesis, Utrecht, 1976 Boelen, H. J. J.: Steroid treatment in sudden idiopathic neural deafness? Arch. Otorhinolaryngol. (N.Y.) 222, 29--34 (1979) Legouix, J. P., Teas, D. C., Beagley, H. A., Remond, M. C.: Relation between the waveform of the cochlea whole nerve action potential and its intensity function. Acta Otolaryngol. (Stockh.) 85, 177-183 (1978) Katsuki, Y., Davis, H.: Electrophysiological studies of ear of Kangaroo rat. J. Neurophysiol. 17, 308-316 (1954) Rosenblith, W. A., Rosenzweig, M. R.: Electrical responses to acoustic clicks: Influence of electrode location in cats. J. Acoust. Soc. Am. 23, 538--588 (1951) Sachs, M. B., Kiang, N. Y.-S.: Two-tone inhibition in auditory nerve fibers. J. Acoust. Soc. Am. 43, 1120-1128 (1968) Sorensen, H.: Auditory adaptation in nerve action potentials recorded from the cochlea in guinea pigs. Acta Otolarnygol. (Stockh.) 50, 438-450 (1959) Spoendlin, H.: Degeneration behaviour of the cochlear nerve. Arch. Otorhinolaryngol. (N.Y.) 200, 275--291 (1971) Teas, D. C., Eldredge, D. H., Davis, H.: Cochlear responses to acoustic transients: An interpretation of whole-nerve action potentials. J. Acoust. Soc. Am. 34, 1438-1459 (1962) Received October 19, 1978
Arch. Otorhinolaryngol. 222, 205-209 (1979)
Oto-Rhino-Laryngology 9 Springer-Verlag1979
The Origin of the Waveform of Cochlear Whole Action Potential H. J. J. Boelen Joris v. d. Haagenlaan 20, Arnhem, The Netherlands Summary. An explanation is given why the recorded action potentials of the cochlea manifest themselves in the complex N1N2-form. The separate AP of the nerve fibers are not diphasic but triphasic in form. With algebraic addition of the, in different phase, AP present in the auditory nerve the N1N 2 complex originates. This is illustrated by the observed form changes in the recorded signals during the cutting experiments of the auditory nerve. Key words: N1N 2 complex of the cochlea
Although action potentials in the cochlear nerve were shown by Buytendijk as early as 1911, there is still no communis opinio as to why the summated AP manifests itself in the characteristic N1N:form. Katsuki and Davis (1954) explained N 2 by the plural response of the nerve: the response with more than one spike of the separate nerve fibers of the cochlear nerve to a presented click. Sorensen (1959) thought that N 2 was mainly formed by the cochlear nucleus in the pons. Rosenblith and Rosenzweig (1951) found a discontinuity in the increase of the amplitude of N 1, recorded from the total cochlear nerve as a function of the rise intensity of the presented signal. They developed the second population theory. Spoendlin (1971) made plausible a different activity of the inner and outer hair cells of the organ of Corti on the basis of his electron microscopic investigations. According to him, the outer hair cells and the afferent nerve fibers connected form part of a very sensitive system, in which summation of pulses of about 10 outer hair cells may occur in one afferent neuron. Sachs and Kiang (1968), however, demonstrated that all afferent nerve fibers of the cochlear nerve recorded by them reacted to stimuli being close to the auditory threshold as to their level of intensity. Teas et al. (1962) and Legouix et al. (1978) presented a model of the AP to explain how diphasic neural elements of the population of the fibers could combine
0302-9530/79/0222/0205/$ 01.00
206
H.J.J. Boelen
in amplitude and phase as a travelling wave excites the elements successively to produce the classical N 1 and N 2 deflections. Boelen (1976), however, demonstrated that the AP in the separate nerve fibers of the cochlear nerve are not diphasic but triphasic. In measurements at the auditory nerve with guinea pigs there is still found a positive peak Pl before the first negative deflection N 1. This P1 is the first phase of the AP "the approaching wave" and it is formed by the positive mutation of polarity in the nerve as an introduction of the negatively directed membrane shifts. Finally, as third phase, the positive recovery phase of the nerve fibers follows. The recorded AP are formed in the well myelinated part of the cochlear nerve. The myelin sheaths of the afferent neurons connected to the inner hair cells already start at the habenula perforata, before the bipolar ganglion cells. The longer the distance covered by the approaching wave in these myelin sheaths up to the recording electrode, the larger the recorded Pj becomes. The P~ is to be recognized clearly in the signals recorded directly from the auditory nerve. A critical examination of the signals recorded from the cochlea shows the PI occasionally, the amplitude of the Pa being much smaller though. At the cochlea recording, the recording electrode is directly situated at the spot of origin of the AP's arising in the underlying turn. Only AP's stimulated by hair cells of cochlea turns situated higher, cover a small distance up to the recording electrode and are able to show some of their approach in the form of a P1. The basilar membrane with human beings is twice as long as it is with guinea pigs, though the part of the auditory nerve situated intracochlear being longer, too. On examination of electrocochleograms recorded with human beings, the P1 "the positive approaching wave" must be taken into account (Boelen, 1979). In order to explain the N1N z complex stimulated by a presented click and recorded from the cochlea, we will have to make use of the following four electrophysiological phenomena: i. the summation of the spikes present in the cochlear nerve; 2. the triphasic phenomenon of the separate spikes with its two positive peaks and one negative deflection; 3. the difference in phase of the spikes present, caused by the travel time of the pulse wave along the basilar membrane; 4. as to the P~, the distance already covered by the several AP in the myelinated nerve fibers up to the recording electrode. The AP recorded in a node of Ranvier of a separate nerve fiber have a size of at least 75 inV. Recordings of the whole cochlear nerve show 0.6 mV after 1000 x preamplification. The algebraic addition of the various positive and negative phases of the AP makes the recorded electric phenomenon more complex but much smaller at the same time. An experiment, in which the auditory nerve of a guinea pig is cut, will demonstrate that the summation of spikes define the form of the N~N 2 complex.
Experiment In an experiment,elaboratelydescr/bedin the afore mentionedpublications,there is a recordingwith guineapigs with two recordingelectrodessimultaneoustyfrom the cochlea and the cochlearnerve. The
The Origin of the Waveform of Cochlear Whole Action Potential
207
cochlea electrode was placed via the opened bulla on the wall of the scala tympani of the second turn. Via a bore hole at the base of the cochlea a tunnel is drilled to the meatus acusticus internus. A needle electrode is placed via this tunnel in the auditory nerve with the aid of a microelectrode holder. Several times this needle electrode is removed and with a fine knife the cochlear nerve can be cut in stages until all nerve fibers have been cut and there can be recorded from the periphere stump only. The earth
-
V
--
i
IO0~V
--
Vv
__
60dB
~
lOOpV
~
,
I
I -
1 0 0 ~V
90dB
200 uv
60 dB
-
--
--
~
.
~
= i .
~
[ 1 0 0 ~V
200 pv
I
5mV
V
-
6OdB
--
5 mV
-1\ -5 mV
I
I
L
--
90dB
--
6
i
;
5
mV
--
i
Fig. 1. Signals recorded from the cochlea and the cochlear nerve during and after the cutting of this nerve. In the figure the signals of the cochlea are represented on the left side, on the right side the accessory signals of the periphere part of the cochlear nerve. Time basis 1 ms, the amplification and the intensity of the presented click are mentioned in the relating signal. Picture 1 taken before the cutting, picture 2 and 3 during the cutting, picture 4 and 5 after eompiete cutting of the nerve, picture 6 indicates the moment before the presented click
208
H.J.J. Boelen
electrode was placed in the neck muscles. During the recordings the experimental animal is presented a click via a condensator telephone every second. The preampliflcation is 1000• In order to get a better survey of the recordings, the recordings of the cochlea and the cochlear nerve, taken simultaneously, are not projected underneath each other but next to each other. Out of the series of signals of a cutexperiment only five pairs of recordings have been pictured (Fig. 1). Viewing the changes in the recorded signals from the cochlea and the cochlear nerve during the cutting of the cochlear nerve we observe: - In the cochlea recording the N: is gradually being taken into the N 1, the basis of the NIN2 complex getting broader. - In the cochlear nerve positive peaks develop, finally the total recorded signal becomes positive. - After complete cutting of the nerve the N1 of the cochlea-recording is strongly enlarged. - The period of time between the beginning of the stimulus and the maximal deflection of the N1 has increased with 0.25 ms.
Discussion After the cutting of the cochlear nerve the maximum amplitude of the N~ in the signal recorded from the cochlea is reached 0.25 ms later than before the cutting. At the same time this amplitude has become 50 times as large at, e.g., a click of 60 dB. The period of time, represented in the basis of the recorded signal, is lengthened as well. These phenomena are to be explained by a strong decrease of the influence of the positive recovery phases of the spikes on the total signal. A n A P is an all or nothing phenomenon. The negative membrane shifts are maximal. After the cutting, however, the recovery phases will pass less quickly in the heavily damaged auditory nerve and thus be directed less strongly positive. The separate nerve fibers are still able to react. The deflection o f the N x, however, formed by the negatively directed membrane shifts originating at the beginning of a click and increasing in number in conformity with the travel time of the pulse wave along the basilar membrane, is later and less strong levelled by the very weakened positive recovery phases of the AP. The N 1 now can manifest itself longer and much more extensively. In the algebraic addition the in amplitude reduced positive recovery phases count less. The N 2 - partially formed by the plural response of the nerve fibers - has been shifted by the extensive N~ and taken into it.
Conclusion With the above mentioned explanation an answer can be given to the origin and the form of the N~N 2 complex "recorded from the cochlea".
Firstly: the
latency period up to the beginning of the N 1 is formed by the period o f time the sound wave needs to reach the hair cells on the beginning of the basilar membrane to be stimulated and by the chemical synaptic transmission between hair cells and afferent neurons.
Secondly: the
two deflections N 1 and N 2 form part o f one big deflection originating by the negative membrane shifts in the afferent neurons of the cochlear nerve in the cochlea. This big deflection will reach its maximum after over 0.25 ms after the
The Origin of the Waveform of Cochlear Whole Action Potential
209
normally found N1 in order to disappear in about 2.5 ms, according to the intensity of the presented click. The soundwave in the scalae needs over 3 ms to reach the hair ceils in the last turn. 1 ms after the beginning of N~ a small increase of negative membrane shifts can be expected, because of the second response of the first neurons after their refractory periods.
Thirdly: because of the contrary influence of the P1 "the positive approaching wave", in the algebraic addition by the recording electrode on the N1, the amplitude of the N~ is represented smaller. The effect of the P~ on the N~ is to be expected only in the second part of the descending line of the N~, when the lower frequency sectors of the basilar membrane are stimulated. N o t until then the action potentials have covered a small distance to the recording electrode and P1 can develop. In a recording of the auditory nerve the P1 will be clearly visible. Fourthly: by
adding the influence of the recovery phases of the directly generated A P to the approaching wave of the action potentials of the lower frequency areas a peak - P2 - originates in the big deflection of the negative membrane shifts between the N 1 and N 2. Owing to this the N 1 and N 2 of the cochlea recording get their characteristic form.
References Boelen, H. J. J.: Her ontstaan van de aktiepotentialen N~ en N 2 in het gehoororgaan, met een verklaring voor her vinden van een P~ in her electrocochleogram. Thesis, Utrecht, 1976 Boelen, H. J. J.: Steroid treatment in sudden idiopathic neural deafness? Arch. Otorhinolaryngol. (N.Y.) 222, 29--34 (1979) Legouix, J. P., Teas, D. C., Beagley, H. A., Remond, M. C.: Relation between the waveform of the cochlea whole nerve action potential and its intensity function. Acta Otolaryngol. (Stockh.) 85, 177-183 (1978) Katsuki, Y., Davis, H.: Electrophysiological studies of ear of Kangaroo rat. J. Neurophysiol. 17, 308-316 (1954) Rosenblith, W. A., Rosenzweig, M. R.: Electrical responses to acoustic clicks: Influence of electrode location in cats. J. Acoust. Soc. Am. 23, 538--588 (1951) Sachs, M. B., Kiang, N. Y.-S.: Two-tone inhibition in auditory nerve fibers. J. Acoust. Soc. Am. 43, 1120-1128 (1968) Sorensen, H.: Auditory adaptation in nerve action potentials recorded from the cochlea in guinea pigs. Acta Otolarnygol. (Stockh.) 50, 438-450 (1959) Spoendlin, H.: Degeneration behaviour of the cochlear nerve. Arch. Otorhinolaryngol. (N.Y.) 200, 275--291 (1971) Teas, D. C., Eldredge, D. H., Davis, H.: Cochlear responses to acoustic transients: An interpretation of whole-nerve action potentials. J. Acoust. Soc. Am. 34, 1438-1459 (1962) Received October 19, 1978
